Simmer is just below bubbles I was taught! I made chicken stock from bones, used my thermometer that was calibrated so I know was working, also got around 204 and the surface just sort of glistens. No bubbles on the surface though!

It's pretty normal. If it's simmering there shouldn't be bubbles breaking the surface. There's no reason to think your thermometer is wrong though.

So... 203 °F = 96 °C...

That's right on the margin where bubbles are really inevitable. It you are heating the water in a very homogeneous way, and has a lid or some salt dissolved, it's possible you don't see any bubble. It's not common, but hey, everybody sees a few uncommon things in their life.

Simmering without bubbles means one of these four:

• You're measuring in Kelvin
• No water and only grease
• Temperature is lower than what you're actually measuring
• Ambient pressure is too high. (You're not in a diving bell, are you?)

Your temperature measurement is probably off. Either that, or the water evaporated long ago and you're now only heating the grease.

My immediate guess is that the measurement is off (measuring pot temp instead of water temp, perhaps) or you're at a really low elevation/there's weirdly high air pressure? The former is far more likely than the latter, usually people only experience water boiling sooner than expected due to high elevation. It should absolutely be simmering at 203

I'd just raise the temp slowly until it starts boiling. The only reason I can think it wouldn't be simmering at that temp is that the water has been boiled off, but that's almost certainly not the case here.

But I've never made stock before.